In a clinical setting, it is often desirable to monitor a patient's health by measuring tissue gas levels. Tissue-gas analyses are an essential part of modern patient care and are used in the diagnosis and treatment of a number of conditions. In particular, measurement of tissue oxygen concentration is heavily relied upon both for general monitoring of overall patient health and for treatment of specific conditions, such as ischemia, burns, and diabetic foot syndrome.
In general, there are three main technologies that are used to perform blood and tissue gas analyses. A first apparatus, known as a pulse oximeter, is a basic, non-invasive instrument that detects hemoglobin saturation. Measuring the percentage of bound hemoglobin provides an estimate of arterial oxygenation. The device may be placed on a finger or another body part, and the measurement is accomplished by monitoring the reflectance or absorbance of incident light. While pulse oximetry is a fast and simple technique, the main drawback is that the measurement does not directly measure arterial oxygen concentrations. In particular, an inaccurate diagnosis may arise is situations where hemoglobin concentrations are low or when hemoglobin is bound to a species other than oxygen.
A second, more direct measurement of tissue gas levels may be made using probe-based systems. Two existing methods based on transcutaneous oxygen (TcpO2) measurement involve either electrodes or an optical sensor foil-based patch. As opposed to a pulse oximeter, TcpO2 measurements can provide a direct indication of microvascular function as TcpO2 maps the actual oxygen supply available for the skin tissue cells. TcpO2 also responds to macrocirculatory events, such as a change in blood pressure.
For the more common electrode based system, the TcpO2 monitor consists of a combined platinum and silver electrode covered by an oxygen-permeable hydrophobic membrane, with a reservoir of phosphate buffer and potassium chloride trapped inside the electrode. A small heating element is located inside the silver anode. In practice, the electrode is applied to an acceptable site on the skin and is heated to 44° C. in order to provide a measurement.
Another commercial system known as the VisiSens™ system combines optical sensor foils with imaging technology. Fluorescent chemical optical sensor foils are attached to the sample surface and read out non-invasively using a microscope. Two-dimensional visualization of oxygen, pH, or CO2 distributions over time can be performed with microscopic resolution.
Limitations of TcpO2 systems include the need for two- (or more) point calibrations with specially prepared, well-defined samples. The sensor must be in contact with the tissue through a contact liquid. If there is air between the tissue and the sensor, the values will be questionable. For commercial electrode systems, it may take 15-20 minutes after the probe has been placed on the skin for the TcpO2 reading to stabilize and it is recommended that calibrations be performed prior to each monitoring period, when changing measuring sites, every four hours, and/or every time an electrode has been remembraned. In addition, heating may affect the ability to acquire physiologically relevant measurements.
A third approach to monitoring gas levels involves a blood test. The test is performed using a blood sample drawn from an artery. The machine used for analysis aspirates this blood from the syringe and measures the pH and the partial pressures of oxygen and carbon dioxide. The bicarbonate concentration can also be calculated. An advantage of the test is that results are usually available for interpretation within minutes. However, the test is invasive, requires a trained practitioner to accurately acquire a sample, and samples must be maintained at room temperature and analyzed quickly or results may be inaccurate.
Overall, while there exists a number of methods for monitoring tissue gas levels, each of these methods possesses inherent drawbacks. It would, therefore, be desirable to have a system and method for monitoring tissue gas levels that (i) is minimally or non-invasive, (ii) is capable of accurately measuring the actual oxygen supply, (iii) provides fast readout, and (iv) requires minimal expertise so as to able to be administered by the patient or other non-practitioner.